1. Field of the Invention
The present invention relates to branching filters and, in particular, to a branching filter including a transmission filter element and a reception filter element that are connected to an antenna terminal.
2. Description of the Related Art
The demand for miniaturization in portable communication terminals is increasing, so it is highly desired to reduce the size of a branching filter for use in a portable communication terminal.
As small branching filters, branching filters including surface acoustic wave (SAW) resonators are widely used. For example, Japanese Unexamined Patent Application Publication No. 2003-249842, which corresponds to U.S. Patent Application Publication No. 2004/0155730, discloses a branching filter that includes a ladder filter element having a plurality of SAW resonators as a transmission filter element and a longitudinally coupled resonator filter element as a reception filter element.
A branching filter must have a transmission filter element having a transmission characteristic (Tx characteristic) of the transmission filter element in which attenuation is large in many frequency bands, including an Rx band, a GPS band, a wireless local area network (WLAN) band, a second-harmonic wave band, and a third-harmonic wave band. One approach to providing attenuation poles in many frequency bands is disclosed in Japanese Unexamined Patent Application Publication No. 2004-173245, which corresponds to U.S. Patent Application Publication No. 2004/0119561. In this approach, attenuation poles are provided by providing an inductor, together with a parallel arm resonator, to a parallel arm in a branching filter using a ladder filter.
Unfortunately, for example, when many attenuation poles are provided by an increased number of parallel arms each having a parallel arm resonator and an inductor, the size of the branching filter is increased. Thus, it is difficult to sufficiently reduce the size of the branching filter.